Plastic lenses are used for various optical applications such as spectacle lenses since plastic lenses have lighter weight, are less easily broken and can be more easily dyed than inorganic lenses.
Typical examples of the material for the plastic lenses include polydiethylene glycol bisallylcarbonate (referred to as CR-39, occasionally).
However, CR-39 has a refractive index of about 1.50 which is smaller than those of inorganic glasses, and a drawback arises in that the thickness of the lens is increased at edge portions in minus-power lenses and at the central portion in plus-power lenses.
Therefore, development of a plastic material having a greater refractive index has been conducted to obtain plastic lenses having a smaller weight and a smaller thickness.
As the process for increasing the refractive index, various processes such as a process of introducing an aromatic ring, a process of introducing a halogen atom other than fluorine atom and a process of introducing sulfur have been proposed.
For example, in Patent Reference 1, a process in which a plastic lens having refractive index of 1.70 or greater and an Abbe number of 35 or greater is obtained by polymerizing a compound having two or more epithio group in one molecule, is disclosed.
In Patent Reference 2, a process in which, in order to further increase the refractive index, a plastic lens having a great refractive index and a great Abbe number is obtained by polymerizing and curing a composition for optical materials comprising a mixture of a compound having sulfur such as an epithio compound and an inorganic compound having sulfur atom and/or selenium atom using a curing catalyst, is disclosed.                In the case of the optical material described in Patent Reference 2, the inorganic compound having sulfur atom and/or selenium atom is frequently a solid component. Therefore, a problem arises in that obtaining a uniform optical material exhibiting excellent transparency is difficult since the compound is separated when the composition is formed or is not completely dissolved when the compound is used in a great concentration.        
In Patent Reference 3, a process in which an optical material exhibiting excellent transparency is obtained by preliminarily polymerizing a composition which comprises prescribed relative amounts by weight of an inorganic compound having sulfur atom and/or selenium atom, an organic compound having sulfur reactive with the inorganic compound such as an epithio compound and a compound promoting the reaction of the inorganic compound and the organic compound having sulfur to obtain a composition for a resin, followed by polymerizing and curing the obtained composition for a resin, is disclosed.    [Patent Reference 1] Japanese Patent Application Laid-Open No. Heisei 9 (1997)-110979    [Patent Reference 2] Japanese Patent Application Laid-Open No. 2001-2783    [Patent Reference 3] Japanese Patent Application Laid-Open No. 2004-197005